Amidst growing trends toward reducing carbon dioxide emissions and depletion of petroleum resources, there is a pressing need for the development of compact vehicles that use only lead-acid batteries and other secondary batteries as motive power sources. In particular, since lead-acid batteries are able to withstand use under harsh conditions and have a suitable weight, they are considered to be useful as, for example, motive power sources of transport vehicles.
Lead-acid batteries undergo electrolysis of water in the electrolyte (sulfuric acid) when overcharged. On the basis of this phenomenon, lead-acid batteries consist of fluid-type lead-acid batteries allow water that has been lost due to electrolysis of water to be replenished, and valve-regulated lead-acid batteries that eliminate the need to replenish water by returning electrolyzed oxygen gas as water. Fluid-type lead-acid batteries have typically been used in the past as motive power sources of forklifts and other transport vehicles due to their suitability to use under harsh conditions despite the need to replenish water.
The configuration of an assembled battery composed of a plurality of fluid-type lead-acid batteries consist of either connecting a plurality of fluid-type lead-acid batteries in series (for example, connecting 60 batteries in series), or connecting series circuits, in which a plurality of fluid-type lead-acid batteries are connected in series, in parallel (for example, connecting 10 series circuits, which connect 6 batteries in series, in parallel).
When considering charging the batteries using the configurations described above, the former configuration is only able to use charging apparatuses having high-voltage specifications for charging, and results in problems such as having to increase the voltage of commercial power supplies. On the other hand, in the case of the latter configuration, a commercial current can be used as is since a charging apparatus having low-voltage specifications can be used for charging. Since fluid-type lead-acid batteries demonstrate a decrease in efficiency when charged at a high hour rate in excess of a 2 hour rate, they use a comparatively small charging current. Thus, in a configuration in which a plurality of series circuits of fluid-type lead-acid batteries are connected in parallel as in the latter configuration, it is comparatively easy to supply charging current in parallel to each series circuit.
In addition, fluid-type lead-acid batteries are less susceptible to the effects of component resistance, and thus in the case of connecting a plurality of fluid-type lead-acid battery series circuits in parallel as in the latter configuration as well, differences in resistance values between each series circuit are small, thereby enabling differences in charging current values distributed to each circuit to be reduced to a negligible level. Consequently, in the case of fluid-type lead-acid batteries, there are few disadvantages to employing the latter configuration, making it more advantageous than the former configuration.
On the other hand, since reactive resistance, which has a greater effect than component resistance, is inversely proportional to electrolyte volume, valve-regulated lead-acid batteries, in which electrolyte volume is lower than that of fluid-type lead-acid batteries, are susceptible to increases in reactive resistance. Consequently, valve-regulated types are more susceptible to variations in internal resistance (component resistance+reactive resistance). Consequently, valve-regulated lead-acid batteries are less suitable for charging in parallel as compared with fluid-type lead-acid batteries.
Thus, fluid-type lead-acid batteries are able to demonstrate favorable charging by charging each fluid-type lead-acid battery in parallel by connecting a plurality of series circuits of fluid-type lead-acid batteries in parallel. Consequently, the use of a method consisting of connecting a charging apparatus to each series circuit while connected in parallel and controlling charging of each series circuit separately (see, for example, Patent Document 1) as a method for charging an assembled battery in which series circuits composed of fluid-type lead-acid batteries are connected in parallel has been avoided since, in addition to being complex, also has the risk of causing variations in charging capacity between each series circuit due to errors within the rated current range of each charging apparatus.    Patent Document 1: Japanese Patent Application Laid-open No. S55-053140